Since you will have a certain "long" time in Xi'an, it'll be much more convenient to visit some places which most the tourists won't have the chance to go.
The Zhong Nan mountain is part of the Qin Ling, which is the geographical watershed of the north China and South China. There are many forest reserve under the national protection. And the hotspring in Shaanxi Province is also very famous, besides the Huaqing Pool in Xi'an, there are many hotsprings in the city around Xi'an, just like the He Chuan hotspring in HanCheng city.
As one of the oldest city in China, Xi'an had been found as the capital for a duration about more than 1500 years, so the mausoleums of the ancient emperors are worth visiting too.
It's hard to introduce you all the places in such a limited space, but it's really deserve to come and visit.